1. Field of the Invention
The present invention relates to a pouch-type rechargeable battery and its method of manufacture, and more particularly, the present invention relates to a pouch-type rechargeable battery using a pouch as an external case in housing an electrode assembly.
2. Description of Related Art
In general, a lithium rechargeable battery uses a non-aqueous electrolyte due to the reactivity of lithium with water. The non-aqueous electrolyte may be a solid polymer containing a lithium salt or a liquid in which of a lithium salt is dissociated in an organic solvent. Lithium rechargeable batteries can be classified into lithium metal batteries and lithium ion batteries, which use liquid electrolytes, and lithium ion polymer batteries which uses a polymer electrolyte, depending upon the types of electrolyte.
A problem of leakage of a lithium ion polymer battery, in comparison with a lithium ion battery using liquid electrolytes, does not occur. Accordingly, the lithium ion polymer battery can use a multi-layered pouch including a metal foil and one or more polymer membranes covering the top and bottom surfaces of the metal foil instead of a metal can used in the lithium ion battery.
When the multi-layered pouch uses lithium rechargeable batteries, it is possible to reduce the weight and thickness of the battery, and to relatively freely change the shape of the battery, as compared with a battery in which a metal can is used.
The conventional pouch for a lithium ion polymer battery has a multi-layered structure by sequentially stacking a polyolefin resin layer, a thermal adhesive layer, which acts as a sealing material with a thermal adhesive property, a metal foil layer, such as aluminum or the like maintaining mechanical strength, and a nylon layer which act as a protection layer. CPP (Casted Polypropylene) is normally used as the polyolefin resin layer.
In a general method of assembling a pouch-type lithium rechargeable battery, a middle portion of a rectangular pouch membrane is folded to form the top and bottom of the pouch. A drawing part, which can receive an electrode assembly or the like, is formed by a press processing in the bottom of the pouch. The formed drawing part makes it easy to install the electrode assembly in the post-processes, thereby making it easy to form a flange on the flange part of the pouch on the basis of the drawing part, and the flange part forms a sealing part in the following sealing processes. In addition, the sealing part can be folded and arranged, thereby compactly forming the pouch.
An electrode assembly, which is formed by sequentially stacking a positive electrode plate, the first separator, a negative electrode plate, and the second separator, is wound in a spiral form to be formed as a jelly roll. The formed jelly roll is disposed in the drawing part of the bottom of the pouch. The flange part, which has a shape of a flange, is heated and pressed to form a bare cell of a battery while the flange parts of the top and bottom of the pouch are brought into tight contact with each other.
In order to electrically connect a positive electrode plate and a negative electrode plate of an electrode assembly to outside of the pouch, electrode tabs are formed on one side of each of a positive electrode plate and a negative electrode plate. These electrode tabs are formed to be projected from the jelly roll in a direction perpendicular to the winding direction of the jelly roll and are drawn out through one side of the pouch to be sealed.
Accessories or structures, such as a Protective Circuit Module (PCM) or a Positive Temperature Coefficient (PTC) element, are attached to the bare cell, of which the pouch has been sealed, to form a core cell. Thereafter, the core cell is inserted and combined into a hard case to form a hard pack.
In the end of the flange part of the pouch, a metal foil constituting the middle layer of a pouch membrane has been exposed. Accordingly, when the Protective Circuit Module is combined with the pouch-type bare cell, the conductive part of the Protective Circuit Module is very likely to be electrically connected to the exposed metal foil. If the conductive part of the Protective Circuit Module is directly or indirectly connected to the electrode of the bare cell, it is more likely that the electrode and the metal foil of the pouch membrane are electrically connected.
Erosion of the metal foil of the pouch membrane, which is made of aluminum or the like by an electrical and chemical action, can occur if the metal foil of the pouch membrane is electrically connected to the electrode. In particular, if electrolyte ingredients or humidity exist around the electrode tabs of the pouch, the erosion of the metal foil is accelerated.
Since the metal foil, which acts as a barrier between water and oxygen, is continually eroding, only a polymer layer of the pouch membrane is not enough to block the inflow of water and oxygen. In addition, as exterior water or oxygen flows therein, abnormal phenomenon, such as swelling, occurs to cause abandonment, performance degradation of the battery, and shortened life time of the battery.
In the process of sealing the pouch, a predetermined ingredient may be added to a surface of a polymer layer for reinforcing the bonding between the polymer layer, such as CPP, inside the pouch and a metal including the electrode tab. In addition, a resin insulation layer, such as an insulation tape, may be further attached to the electrode tab for preventing an occurrence of a short circuit between the electrode tab and the metal foil of the pouch.
Contemporary designs for pouch-type rechargeable batteries tend to ineffectively utilize the space and capacity of the batteries, and thus suffer from reduced charging and discharging capacity, with an accompanying reduction in capacity-to-volume, or capacity-to-width ratio.